1. Field of the Invention
The invention relates to polyolefin-based adhesive blends suitable for adhering fluoropolymers. More specifically, the adhesive compositions are useful for adhering fluoropolymers to propylene polymers and are blends of linear low density polyethylene, plastomer, elastomeric material and modified polyolefin. The invention also provides composite structures comprising a propylene polymer layer, a poly(vinylidene difluoride) layer and an intervening layer of the adhesive composition.
2. Description of the Prior Art
Poly propylene (PP) resins are widely used by the automotive industry for producing larger parts such as bumpers, body side moldings and the like. While advances have been made in optimizing the physical characteristics of these products, paint adhesion continues to be a problem. Even when suitable finishes are developed for spray application, the process is labor intensive, requiring preparation of the surface of the formed part by priming or the like and the application of one or more coats of the paint finish.
Pigmented thin films cast from poly(vinylidene difluoride) (PVDF) dispersions are increasingly being used by the auto industry for the manufacture of colored trim parts. This process involves thermally laminating a PVDF-based paint film onto a polyolefin sheet, generally a polypropylene-based thermoplastic polyolefin (TPO). The multi-layer sheet is then thermoformed to the desired shape. There is, however, still a problem with this method since PVDF films, like the spray paints they replace, have poor adhesion to propylene polymers. An adhesive or tie layer between the propylene polymer and PVDF layer is necessary to bond the materials.
Chlorinated polyolefins, such as chlorinated polyethylene, have been used to bond PVDF and PP but their use is limited to thermal lamination processes since chlorinated polyethylenes do not flow well at the low shear rates used for coextrusion processes and they are unstable at the high temperatures required for processing PP. In this process a sheet or film of chlorinated polyethylene is separately formed and subsequently sandwiched between layers of PVDF and PP sheet or film and the multi-layer construction thermally laminated to bond the layers together. The resulting multi-layer composite is then thermoformed to obtain the finished part. This procedure is not totally satisfactory since chlorinated polyethylene can evolve hydrogen chloride (HCl) during one or both of the fabrication operations, depending on the temperatures being used.
Polyurethanes and polyacrylics, such as poly(methylmethacrylate), are known adhesives for PVDF. Polyolefins, particularly ethylene-vinyl acetate (EVA) copolymers, are also known. The vinyl acetate (VA) content of useful EVAs tends to be high which substantially limits their utility with PP because they tend to degrade at the high temperatures, typically greater than 450° F., used to process PP. This results in the undesirable liberation of acetic acid and water and the presence of black specks (decomposed polymer).
Dehennau (U.S. Pat. No. 4,959,269) teaches that EVA copolymers (>36% VA) adhere to PVDF but do not adhere as well as the urethanes and acrylates. FR-A-1,484,153 describes the use of EVAs containing 13 to 35% VA for bonding polyolefins to a fluorinated hydrocarbon polymer, including PVDF. Decroly (U.S. Pat. No. 4,659,625) teaches that EVAs containing at least 50% VA bond to PVDF as well as to polar polymers such as polyesters, acrylics, nylons, etc. Kim (U.S. Pat. No. 5,139,878) describes the use of a blend of an anhydride-modified polyolefin and an ethylene-alkyl ester copolymer, such as ethylene-methyl acrylate (EMA), as an adhesive for PVDF. DeAntonis (U.S. Pat. No. 4,677,017) teaches that PVDF can be bonded to polyolefins by using an adhesive polymer blend consisting of a modified EVA (modified with carboxylic acid anhydrides, for example) and an EVA copolymer. Other references which relate to the application of fluoropolymer paint and decorative films on substrates include U.S. Pat. Nos. 4,810,540, 4,943,680 and 6,251,506 and PCT published application (WO 94/0337).
There is a continuing need for adhesives for bonding fluoropolymers. It would be even more advantageous if inexpensive polyolefin adhesive blends which could be coated directly onto PP or coextruded with the PP and used to bond with PVDF films were available.